Methane emissions from various sources represent a significant portion of non-CO2 greenhouse gas emissions. Traditionally, coal mine and landfill operators have been able to recover only a small percentage of dilute methane gas streams because they contain significant quantities of contaminants, such as CO2, oxygen, water vapor, and nitrogen. The cost of conventional gas separation systems, in particular nitrogen removal units, represents one of the most significant hurdles to mitigating non-CO2 greenhouse emissions. Removal of methane from sub-quality sources has the potential to reduce annual greenhouse gas emissions by about 23.5 billion equivalent kilograms of CO2 and to enable the cost-effective recovery of about 3.5 trillion cubic feet per year of natural gas. This represents a reduction of about 0.3% in annual U.S. greenhouse gas emissions at no net cost when the value of pipeline quality natural gas is realized.
Four commercial methods are currently used to remove nitrogen from natural gas: cryogenic distillation, pressure swing adsorption (PSA), lean oil absorption, and membrane separation. Cryogenic distillation involves the condensation of dry natural gas, followed by distillation of nitrogen at very low temperatures (e.g., about −150° C.). This technique is used commercially to separate nitrogen from natural gas. Although methane recovery is high, there is a significant pretreatment cost (water and CO2 removal). In addition, the complexity of the system makes reliability an issue. Cryogenic distillation is generally more cost-effective at large capacities (e.g., about 75 million standard cubic feet per day (MMSCFD)), while most coal mine and landfill opportunities are typically in the 2-10 MMSCFD capacity range. Cryogenic distillation also requires significant energy to compress the gas stream.
In most conventional PSA processes, methane is selectively adsorbed onto carbon sieves, leaving nitrogen in the raffinate. Pretreatment and multiple beds are required, which leads to high capital costs. This method also requires methane to be recompressed and is inflexible to variations in flow rates. PSA has been used on a limited commercial basis for nitrogen separation and is best suited for low (e.g., about 2 to 10 MMSCFD) gas flow rates and high nitrogen content.
The lean oil absorption process involves the absorption of methane in chilled hydrocarbon oil. This process is energy-intensive and, therefore, has high processing costs. In addition, the large equipment used for this process makes redeployment unlikely.
Membrane separation involves separating nitrogen from natural gas by a process wherein the methane selectively permeates through a membrane, and the raffinate is a nitrogen-rich stream that can be burned to run a permeate compressor. Membranes have a low methane recovery (e.g., about 80%) that makes them less attractive.